1. Field of the Invention
The invention relates to the field of assemblies of laser diode arrays and their cooling systems.
The fields of application of assemblies of laser diode arrays are numerous and can be grouped into two major families:
applications where the laser radiation of the diodes is used directly (beams shaped by optics or carried by fibres) and essentially relating to the medical sector and the industrial sector (marking); PA1 applications where the laser radiation of the diodes is used for the optical pumping of laser media (energy storage media or nonlinear effect media), e.g. the optical pumping of crystalline materials, such as oxides or fluorides doped with ions of rare earths (Nd, Er, Tm, Ho, Yb, Pr, Ce, . . . ) or Cr, Co and similar ions, optical pumping of laser material, such as dyes and the interaction with nonlinear effect media (generation of harmonics, parametric amplifiers or oscillators). PA1 a rigid, mechanical support function, PA1 an electrical function (anode), PA1 a thermal function, due to a high thermal conductivity. PA1 each diode is assembled on the end of a base forming an anode and traversed by a hole, all the holes defining a channel for the circulation of a fluid for cooling the linear assembly of diode arrays, each module, constituted by an array, the base and a cathode, being separated from each immediately adjacent module by a flat joint, PA1 each base being provided with a second hole, all the second holes being aligned when the arrays are juxtaposed in the operating position, PA1 two holding flanges are placed at the ends of the linear assembly of arrays, said flanges being tightened by a screw, which passes through the assembly through all the second holes.
2. Discussion of the Background
Laser diode arrays and in more general terms laser diodes, have an optical/electrical efficiency of approximately 25 to 33%. Consequently the power lost by heating represents 66 to 75% of the consumed electrical power. In a continuous 20 Watt array, the active volume (semiconductor) is generally very small (micrometric dimensions) and typically 0.5 to 1 mm.sup.3, whilst the thermal power density is between 60 and 160 kW/cm.sup.3. The array is generally welded to a base (made from brass or copper), which has several functions:
In all commercial laser diode arrays, heat conduction takes place from the bottom, through the substrate and in general the anode base. This base must be in contact with a cold source in order to remove the thermal power and limit the temperature rise in the semiconductor. The connection in the upper part (cathode) cannot generally ensure an adequate thermal conductivity and the heat exchange takes place by natural convection of the ambient air.
In order to create a cold source, it is either possible to use a metal (brass or copper) cold box, or a Peltier effect thermocouple.
These procedures assume a good contact between the base of the array and the cold source, in order to minimize the contact thermal resistivity. The Peltier effect technology is very widely used in low power applications and when there is only a limited number of diodes. There is no example of the use of this technology for continuous power arrays (typically 21 Watt on average).
A so-called "microchannel" technology is described in U.S. Pat. No. 5,105,429 and U.S. Pat. No. 5,105,430. This technology is based on the assembly of modules in order to form a bidimensional emitting structure, each module having a diode array placed on a substrate with microchannel cooling.
Improvements or variants have been made, relating to the internal architecture of the structure, the construction of the stacks or the way in which the cooling fluid circulates. The principle of cooling by a microchannel structure remains the same, there being the benefit of a large exchange surface (radiator) by having a network of fins in a conductive material. The material can be the actual semiconductor or a material having a good heat conduction, such as silicon, copper or diamond.
The microchannel radiator can be integrated into each array base in order to have an autonomous module, or can be common to assemblies of arrays (surface of bidimensional assembly).
The field of application of microchannel technology is that of stacks of arrays. 1 cm wide or wider array stacks make it possible to obtain emissive areas of several square centimeters. This is the most widely used alternative in connection with surface emission diode networks. For the reasons indicated below, the development of the microchannel method is very delicate.
Firstly, this microchannel method is only accessible by highly sophisticated etching technologies. The size of the channels is also approximately 100 microns, with a spacing of 50 to 150 microns. Therefore the cooling liquid must be filtered with great care so as not to clog the network of microchannels. This leads to a significant pressure loss which, for reasonable fluid flow rates of approximately one liter per minute, assume high pressures (exceeding 4 bars).
Finally, in the case of optical pumping, the known stacks of microchannel arrays do not make it possible to pump media, e.g. having a cylindrical shape. A more conventional technology consists of assembling the arrays in stack form and cooling them from the rear in contact with a water box. This technology can only be envisaged for quasi-continuous emission arrays, i.e. for average powers of 1 to 4 Watts and remains the solution proposed on commercial products.
U.S. Pat. No. 5,031,184 describes a semiconductor pumping diode cooling device. This device has a complex structure, because it involves an assembly on Peltier elements, which themselves rest on a base.
No information is given concerning the assembly of the system and particularly the retention of the parts and joints with respect to the base.
In such a device, it would appear to be difficult to be able to guarantee the sealing and mechanical strength of the system, particularly as a positioning from below and the two sides is difficult to implement. Moreover, thermal expansions may give rise to mechanical stresses which are difficult to contain within such an assembly.
From the thermal standpoint, it would appear to be very difficult with the device of U.S. Pat. No. 5,031,184 to finally regulate the temperature of the semiconductors by means of Peltier elements, when the latter are sandwiched between two water cooling systems, whose function is to ensure a good heat removal due to a good heat exchange coefficient. Although this principle may be usable for a single element it cannot be used for two elements, because the effect of the first Peltier element would have a consequence on the second element, making the second Peltier element compensate the defect, giving rise to the same consequences on the third element and so on.